Leak-compensated proportional assist ventilation

ABSTRACT

This disclosure describes systems and methods for compensating for leakage when during delivery of gas from a medical ventilator in a proportional assist mode to a patient. The technology described herein includes systems and methods to compensate the delivery of PA ventilation for leakage in the patient circuit by using leak-compensated lung flows as well as leak-compensated respiratory mechanics parameters (lung compliance and lung resistance) estimated in a manner that compensates for elastic and inelastic leaks from the ventilation system.

INTRODUCTION

In mechanical ventilation, proportional assist (PA) refers to a type ofventilation in which the ventilator acts as an inspiratory amplifierthat provides pressure support based on the patient's effort. The degreeof amplification (the “support setting”) is set by an operator, forexample as a percentage based on the patient's effort. In oneimplementation of PA ventilation, the ventilator may continuouslymonitor the patient's instantaneous inspiratory flow and instantaneousnet lung volume, which are indicators of the patient's inspiratoryeffort. These signals, together with ongoing estimates of the patient'slung compliance and lung resistance, allow the ventilator to compute theinstantaneous pressure at a point in the ventilation circuit thatassists the patient's inspiratory muscles to the degree selected by theoperator as the support setting.

PA ventilation relies on certain physiological principles. The act ofinspiration requires the patient's inspiratory muscles to develop apressure gradient between the mouth and the alveoli sufficient to drawin breathing gas and inflate the lungs. Some of this pressure gradientis dissipated as gas travels through the artificial airway and thepatient's conducting airways, and some of the pressure gradient isdissipated in the inflation of the lungs and thorax. Each element ofpressure dissipation is characterized by a measurable property: theresistance of the artificial and patient airways, and the compliance (orelastance) of the lung and thorax.

The ventilator providing PA ventilation uses specific information,including resistance of the artificial airway, resistance of thepatient's airways, lung compliance, instantaneous inspiratory flow andnet lung volume, and the support setting to compute the instantaneouspressure to be applied at the wye. The ventilator may estimate patientlung resistance and lung compliance, for example approximately everyfour to ten breaths. In one implementation, every computational cycle(e.g., 5 milliseconds), the ventilator estimates lung flow, based on anestimate of circuit flow, and lung volume, based on the integral valueof estimated circuit flow.

PA ventilation begins inspiratory assist when flow (generated by thepatient's inspiratory muscles) is detected. If the patient ceasesinspiration, the assist also ceases. Once inspiratory flow begins, theventilator monitors instantaneous flow and volume and applies thepressure calculated to deliver a proportion (determined by the supportsetting) of the total demand as determined by the patient's inspiratoryeffort. In Tube Compensation (TC) ventilation, the ventilator monitorsinstantaneous flow and volume and applies the pressure calculated toovercome a proportion (determined by the support setting) of thepressure losses dissipated across the resistance of the artificialairways (e.g., endotracheal tube).

The lung compliance and lung resistance of a patient may be collectivelyreferred to as the respiratory mechanics of the lung or, simply, thepatient's respiratory mechanics. Because PA relies on the patient'srespiratory mechanics, more accurate determination of respiratorymechanics is essential to performance of the ventilator when providingPA.

Leak-Compensated Proportional Assist Ventilation

This disclosure describes systems and methods for compensating forleakage when during delivery of gas to a patient from a medicalventilator in a proportional assist (PA) mode. The technology describedherein includes systems and methods that compensate the delivery of PAventilation for leakage in the patient circuit by using leak-compensatedlung flows as well as respiratory mechanics (lung compliance and lungresistance) estimated in a manner that compensates for elastic andinelastic leaks from the ventilation system.

In part, this disclosure describes a method of compensating for leakagein a ventilation system during delivery of gas from a medical ventilatorproviding PA to a patient. The method includes monitoring aninstantaneous flow in the ventilation system based on one or moremeasurements of pressure and flow in ventilation system. Leakage fromthe system is modeled as a first leakage component through a firstorifice of a fixed size and a second leakage component through a secondorifice of a varying size, in which the first and second leakagecomponents are different functions of instantaneous pressure in theventilation system. A leak-compensated instantaneous lung flow of gasinhaled or exhaled by the patient is estimated based on the one or moremeasurements, the first leakage component and second leakage component.The leak-compensated lung flow and a predetermined respiratory mechanicsmodel are used to estimate a leak-compensated lung compliance and aleak-compensated lung resistance. A pressure to be delivered to thepatient is then calculated based on the leak-compensated lung flow, theleak-compensated lung compliance and the leak-compensated lungresistance.

This disclosure describes a method of compensating for leakage in aventilation tubing system during delivery of gas from a medicalventilator to a patient. The method includes receiving a support settingidentifying an amount of proportional assistance to provide to thepatient. An inelastic leakage in the ventilation system is identified asa first function of at least one of a pressure measurement and a flowmeasurement in the ventilation system. An elastic leakage in theventilation system is also identified as a second function of at leastone of the pressure measurement and the flow measurement in theventilation system. The circuit compliance and circuit resistance of theventilation tubing system is then used along with estimated lungcompliance of the patient and estimated lung resistance of the patientbased on the inelastic leakage, the elastic leakage, the circuitcompliance, circuit resistance and the at least one of the pressuremeasurement and the flow measurement in the ventilation system.Ventilation is then delivered to the patient based on estimated patienteffort and the support setting, in which the patient effort isdetermined from estimated lung flow using the inelastic leakage, theelastic leakage, the lung compliance and the lung resistance.

The disclosure also describes a pressure support system that includes apressure generating system adapted to generate a flow of breathing gasand a ventilation tubing system including a patient interface device forconnecting the pressure generating system to a patient. One or moresensors are operatively coupled to the pressure generating system or theventilation system, in which each sensor capable of generating an outputindicative of a pressure of the breathing gas. A leak estimation moduleis provided that identifies leakage in the ventilation system andcompensates the calculation of lung flow for the estimated leakage inthe system. A respiratory mechanics calculation module is furtherprovided that generates a leak-compensated lung compliance and aleak-compensated lung resistance based on the leakage and at least oneoutput indicative of a pressure of the breathing gas. The system furtherincludes a proportional assistance ventilation module that causes thepressure generating system to provide ventilation to the patient basedon patient effort and a support setting, in which the patient effort isdetermined from estimated lung flow using the leakage, theleak-compensated lung compliance and the leak-compensated lungresistance.

The disclosure further describes a PA ventilation controller for amedical ventilator. The controller includes a microprocessor, a modulethat compensates calculations of lung compliance and lung resistancebased on instantaneous elastic leakage and instantaneous inelasticleakage of breathing gas from a ventilation system, and a pressurecontrol module that provides proportional assist ventilation based onthe compensated lung compliance and lung resistance.

These and various other features as well as advantages whichcharacterize the systems and methods described herein will be apparentfrom a reading of the following detailed description and a review of theassociated drawings. Additional features are set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the technology. Thebenefits and features of the technology will be realized and attained bythe structure particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arcintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawing figures, which form a part of this application,are illustrative of described technology and are not meant to limit thescope of the invention as claimed in any manner, which scope shall bebased on the claims appended hereto.

FIG. 1 illustrates an embodiment of a ventilator connected to a humanpatient.

FIG. 2 schematically depicts example systems and methods of ventilatorcontrol.

FIG. 3 illustrates an embodiment of a method of compensating for leakagein a ventilator providing pressure assistance to a patient.

FIG. 4 illustrates an embodiment of a method for providing aleak-compensated PA breath to a patient.

FIG. 5 illustrates an embodiment of a method for estimating respiratorymechanics of patient that utilizes a respiratory mechanics maneuver.

FIG. 6 illustrates an embodiment of a method for dynamically estimatingrespiratory mechanics of patient.

FIG. 7 illustrates a functional block diagram of modules and othercomponents that may be used in an embodiment of ventilator thatcompensates for elastic and rigid orifice sources of leaks whenperforming PA ventilation.

DETAILED DESCRIPTION

Although the techniques introduced above and discussed in detail belowmay be implemented for a variety of medical devices, the presentdisclosure will discuss the implementation of these techniques in thecontext of a medical ventilator providing pressure assist (PA)ventilation to a human patient. The reader will understand that thetechnology described in the context of a medical ventilator for humanpatients could be adapted for use with other systems such as ventilatorsfor non-human patients and general gas transport systems in which leaksmay cause a degradation of performance.

FIG. 1 illustrates an embodiment of a ventilator 20 connected to a humanpatient 24. Ventilator 20 includes a pneumatic system 22 (also referredto as a pressure generating system 22) for circulating breathing gasesto and from patient 24 via the ventilation tubing system 26, whichcouples the patient to the pneumatic system via physical patientinterface 28 and ventilator circuit 30. Ventilator circuit 30 could be adual-limb or single-limb circuit for carrying gas to and from thepatient. In a dual-limb embodiment as shown, a wye fitting 36 may beprovided as shown to couple the patient interface 28 to the inspiratorylimb 32 and the expiratory limb 34 of the circuit 30.

The present systems and methods have proved particularly advantageous innoninvasive settings, such as with facial breathing masks, as thosesettings typically are more susceptible to leaks. However, leaks dooccur in a variety of settings, and the present description contemplatesthat the patient interface may be invasive or non-invasive, and of anyconfiguration suitable for communicating a flow of breathing gas fromthe patient circuit to an airway of the patient. Examples of suitablepatient interface devices include a nasal mask, nasal/oral mask (whichis shown in FIG. 1), nasal prong, full-face mask, tracheal tube,endotracheal tube, nasal pillow, etc.

Pneumatic system 22 may be configured in a variety of ways. In thepresent example, system 22 includes an expiratory module 40 coupled withan expiratory limb 34 and an inspiratory module 42 coupled with aninspiratory limb 32. Compressor 44 or another source(s) of pressurizedgas (e.g., air and oxygen) is coupled with inspiratory module 42 toprovide a gas source for ventilatory support via inspiratory limb 32.

The pneumatic system may include a variety of other components,including sources for pressurized air and/or oxygen, mixing modules,valves, sensors, tubing, accumulators, filters, etc. Controller 50 isoperatively coupled with pneumatic system 22, signal measurement andacquisition systems, and an operator interface 52 may be provided toenable an operator to interact with the ventilator (e.g., changeventilator settings, select operational modes, view monitoredparameters, etc.). Controller 50 may include memory 54, one or moreprocessors 56, storage 58, and/or other components of the type commonlyfound in command and control computing devices.

The memory 54 is computer-readable storage media that stores softwarethat is executed by the processor 56 and which controls the operation ofthe ventilator 20. In an embodiment, the memory 54 comprises one or moresolid-state storage devices such as flash memory chips. In analternative embodiment, the memory 54 may be mass storage connected tothe processor 56 through a mass storage controller (not shown) and acommunications bus (not shown). Although the description ofcomputer-readable media contained herein refers to a solid-statestorage, it should be appreciated by those skilled in the art thatcomputer-readable storage media can be any available media that can beaccessed by the processor 56. Computer-readable storage media includesvolatile and non-volatile, removable and non-removable media implementedin any method or technology for storage of information such ascomputer-readable instructions, data structures, program modules orother data. Computer-readable storage media includes, but is not limitedto, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memorytechnology, CD-ROM, DVD, or other optical storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which can be used to store the desired informationand which can be accessed by the computer.

As described in more detail below, controller 50 issues commands topneumatic system 22 in order to control the breathing assistanceprovided to the patient by the ventilator. The specific commands may bebased on inputs received from an operator, the patient 24, the pneumaticsystem 22 and sensors, the operator interface 52 and/or other componentsof the ventilator. In the depicted example, operator interface includesa display 59 that is touch-sensitive, enabling the display to serve bothas an input and output device.

FIG. 2 schematically depicts exemplary systems and methods of ventilatorcontrol. As shown, controller 50 issues control commands 60 to drivepneumatic system 22 and thereby circulate breathing gas to and frompatient 24. The depicted schematic interaction between pneumatic system22 and patient 24 may be viewed in terms of pressure and/or flow“signals.” For example, signal 62 may be an increased pressure which isapplied to the patient via inspiratory limb 32. Control commands 60 arebased upon inputs received at controller 50 which may include, amongother things, inputs from operator interface 52, and feedback frompneumatic system 22 (e.g., from pressure/flow sensors) and/or sensedfrom patient 24.

In an embodiment, before the respiratory mechanics of a patient can bedetermined, the mechanics of the ventilation tubing system may bedetermined. For example, when modeling the delivery of gas to and from apatient 24 via a closed-circuit ventilator, one simple assumption isthat compliance of the ventilator circuit 30 (the “circuit compliance”)is fixed and that all gas injected into the ventilator circuit 30 thatdoes not exit the circuit 30 via the expiratory limb 34 (in a dual-limbembodiment) fills the circuit as well as the patient's lungs and causesan increase in pressure. As gas is injected (L₁), the lung responds tothe increased gas pressure in the circuit 30 by expanding. The amountthe lung expands is proportional to the lung compliance and is definedas a function of gas pressure differential (e.g., lung compliance=volumedelivered/pressure difference). As discussed in greater detail below,this assumption is not valid when leaks are present.

The term circuit compliance is used to refer to the relationship betweenthe amount the pressure in the ventilator circuit 30 (or ventilatorcircuit 30 and attached patient interface 28, depending on how thecompliance is determined) and changes in volume delivered into thecircuit. In an embodiment, the circuit compliance may be estimated bypressurizing the ventilator circuit 30 (or circuit 30 and interface 28combination) when flow to the patient is blocked and measuring thevolume of additional gas introduced to cause the pressure change(compliance=volume delivered/pressure difference).

The term circuit resistance is used to refer to the amount the pressurechanges between two sites upstream and downstream the ventilator circuitas a function of volumetric flow rate through that circuit. Circuitresistance may be modeled as a two-parameter function of flow andseveral methods for modeling and calculating circuit resistance areknown in the art. For example, in an embodiment, the circuit resistancemay be estimated by passing several fixed flow rates through the circuitand measuring the pressure difference between certain upstream anddownstream sites and finding the best curve fit to the collected data.

Methods of determining circuit compliance and circuit resistance (suchas those described above) may be executed by the operator prior toattaching the patient to the ventilator as part of the set up of theventilator 20 to provide therapy. Other methods of determining circuitcompliance and/or resistance are also known and could be adapted for usewith the disclosed leak-compensation systems and methods describedherein.

In many cases, it may be desirable to establish a baseline pressureand/or flow trajectory for a given respiratory therapy session. Thevolume of breathing gas delivered to the patients lung (L₁) and thevolume of the gas exhaled by the patient (L₂) are measured ordetermined, and the measured or predicted/estimated leaks are accountedfor to ensure accurate delivery and data reporting and monitoring.Accordingly, the more accurate the leak estimation, the better thebaseline calculation of delivered and exhaled flow rates and volumes.

Errors may be introduced due to leaks in the ventilation tubing system26. The term ventilation tubing system 26 is used herein to describe theventilator circuit 30, any equipment attached to or used in theventilator circuit 30 such as water traps, monitors, drug deliverydevices, etc. (not shown), and the patient interface 28. Depending onthe embodiment, this may include some equipment contained in theinspiration module 42 and/or the expiration module 40. When referring toleaks in or from the ventilation tubing system 26, such leaks includeleaks within the tubing system 26 and leaks where the tubing system 26connects to the pressure generator 22 or the patient 24. Thus, leaksfrom the ventilation tubing system 26 include leaks from the ventilatorcircuit 30, leaks from the patient interface 28 (e.g., masks are oftenprovided with holes or other pressure relief devices through which someleakage may occur), leaks from the point of connection of the patientinterface 28 to the patient 24 (e.g., leaks around the edges of a maskdue to a poor fit or patient movement), and leaks from the point ofconnection of the patient interface 28 to the circuit 30 (e.g., due to apoor connection between the patient interface 28 and the circuit 30).

For the purpose of estimating how a leak flow rate changes based onchanges in pressure in the ventilation tubing system 26, theinstantaneous leak may be modeled as a leak through a single rigidorifice or opening of a fixed size in which that size is determinedbased on comparing the total flow into the inspiratory limb 32 and outof the expiratory limb 34. However, this leak model does not take intoaccount any elastic component of leak source(s) in the system 26, thatis how much of the area of any of the holes or openings in theventilation tubing system 26 through which leakage occurs may change dueto an increase or decrease in pressure.

It has been determined that not accounting for elastic leakage from theventilation tubing system 26 can cause many problems. First, if only theinelastic/fixed orifice model is used to estimate leak, the subsequenterrors caused by ignoring the elastic effects of any actual leaks end upgenerating inaccurate estimates of flow rates into the lung. This cancause the ventilator 20 to estimate gas volume delivered into the lunginaccurately when, in fact, the elastic leaks in the system 26 have letmore gas escape than estimated. Second, if the elasticity of the leaksource is ignored, any other calculation, estimate, or action that theventilator 20 may perform which is affected by the leak estimate will beless accurate.

In the systems and methods described herein, the provision of PAventilation is made more accurate by compensating for tubing systemleakage. In the embodiments described herein fixed (rigid) and elasticcomponents of the system leakage are used when determining the lungflow, net lung volume, lung compliance and lung resistance of thepatient. This results in a more accurate determination of lungcompliance and lung resistance and, therefore, ventilation of patientsbased on respiratory mechanics. While the systems and methods arepresented in the context of specific leakage models, the technologydescribed herein could be used to compensate the respiratory mechanicsdetermined by any model for leakage using any type of mechanicalventilator or other device that provides gas.

FIG. 3 illustrates an embodiment of a method of compensating PAventilation for leakage during delivery of gas from a medical ventilatorto a patient. In the method 300 shown, a medical ventilator such as thatdescribed above with reference to FIGS. 1 and 2 is used to provide PAventilation to a patient.

The method 300 illustrated starts with a circuit compliance andresistance operation 302. In that operation 302, the ventilator circuitcompliance and resistance are estimated. In an embodiment, this may beperformed prior to connecting the ventilator to the patient (aspreviously described). Alternatively, it may be dynamically determinedperiodically throughout the delivery of ventilatory support to thepatient.

After the circuit compliance and resistance have been determined, theventilator is connected to the patient and an initialization operation304 is performed. In the initialization operation 304 the ventilatoroperates for an initialization or startup period in order to generate aninitial estimate of lung compliance and lung resistance. If theventilator already has some knowledge of the respiratory mechanics ofthe patient (e.g., the respiratory mechanics have been recentlydetermined during provision of a different type of ventilation or anoperator has provided initial settings for lung compliance andresistance), this operation 304 may be automatically or manually omittedin favor of the previously determined values.

A description of an embodiment of the initialization operation 304 is asfollows. Because the ventilator does not know the patient's mechanicswhen the PA breath type is selected, it performs a startup routine toobtain initial data. In an embodiment, upon startup the ventilatordelivers some number (e.g., two, four, etc.) of consecutive PA breaths,each of which includes an end-inspiratory maneuver that yields estimatesof the patient's resistance and compliance. Using four training breathsfor the initialization operation 304 as an example, the first breath isdelivered using a predicted resistance for the artificial airway andconservative estimates for patient resistance and compliance. Thepredicted values may be determined based on known characteristics of thepatient, such as based on the patient's ideal body weight (IBW), height,gender, age, physical condition, etc. Each of the following three PAbreath's averages stepwise decreased physiologic values with theestimated resistance and compliance values from the previous breath,weighting earlier estimates less with each successive breath, andyielding more reliable estimates for lung resistance and lungcompliance.

In an embodiment of the method 300, a leakage estimate may also be doneprior to the initialization operation 304. Prior determination of leakparameters allows for estimates of respiratory mechanics to be made.This may include delivering pressure-regulated breaths with specificsettings or performing specific “leak maneuvers”, that is a specifiedset of controlled actions on the part of the ventilator that allowleakage parameters to be identified and quantified, such as interruptingthe therapeutic delivery of respiratory gas and holding or changing thepressure and flow, so that data concerning the leakage of the systemduring the controlled actions may be obtained. For example, a leakmaneuver may include periodically holding the pressure and flow in thecircuit constant while determining (based on a comparison of themeasured flow into the inspiratory limb and the measured flow out of theexpiratory limb via the exhalation valve) the net leakage from thesystem. In an embodiment, such a leak maneuver may be performed duringspecific pants of the respiratory phase, e.g., at the end of theexpiratory phase. In yet another embodiment, a sequence ofpressure-based test breaths is delivered with specific settings todetermine leak parameters prior to execution of test breaths forrespiratory mechanics determinations.

After the initialization operation 304, the ventilator provides ongoingPA ventilation to the patient in a PA ventilation operation 306. PAventilation begins inspiratory assist when an inspiratory trigger isdetected such as leak-compensated lung flow (generated by the patient'sinspiratory muscles) exceeds a set sensitivity threshold. If the patientceases inspiration, the assist also ceases. Once inspiratory flowbegins, the ventilator monitors instantaneous flow and volume andapplies the pressure calculated to deliver a proportion (determined bythe support setting) of the total demand as determined by the patient'sinspiratory effort.

During PA ventilation, pressure assistance is provided to the patientbased on the patient's effort and the operator-selected support setting.PA ventilation determines how much to assist the patient's efforts basedon one or more equations of motion used to describe the mechanics of thelung. Such equations use respiratory mechanics of the patient, e.g.,lung resistance and lung compliance, and the circuit when calculatinghow much flow and/or pressure to provide. One example of an equation ofmotion for use in determining the pressure to provide during PAventilation is as follows:

P _(aw)(t)=E∫Qdt+QR−P _(m)(t)

where P_(aw) is the pressure measured at the patient interface, P_(m) isthe pressure generated by the inspiratory muscles of the patient whichis may be used as the index of the patient's effort, E is the lungelastance (which is the inverse of lung compliance, i.e., E=1/C), Q isthe instantaneous leak-compensated lung flow and R is the lungresistance. By manipulating this equation slightly, an equation for theamount of desired pressure assistance at the patient interface toprovide can be obtained:

P _(aw)(t)=kE∫Qdt+kQR

where k is the % support setting based on the patient effort, P_(m). Theequations above are one example of how PA ventilation may be providedbased on patient effort. PA ventilation is known in the art and anysuitable technique or set of equations for determining the assistance toprovide may be used.

As described above, the patient's effort is determined based on thepressure and leak-compensated lung flow in the circuit. In order tocompensate for leakage in the circuit, in the method 300 shown the PAventilation operation 306 includes the ongoing calculation of leakagewhile providing ventilation, as illustrated by the leakagecalculation/compensation operation 307. As discussed in greater detailbelow with reference to FIG. 4, the leakage is calculated and theleak-compensated values for lung flow, current lung volume and netdelivered lung flow may be determined taking into account the calculatedleakage.

The method 300 also includes periodically or randomly performing arespiratory mechanics maneuver in order to recalculate the lungcompliance and lung resistance of the patient. In an embodiment, therespiratory mechanics maneuver operation 308 is performed randomly onceevery four to ten PA breaths.

The respiratory mechanics maneuver operation 308 compensates for leakageas determined by the leakage calculation/compensation operation 307 whencalculating the respiratory mechanics for the patient. This may includestabilizing the pressure and flow in the patient circuit based on thecalculated leakage at that pressure so that the flow into the patientcircuit is approximately equal to the calculated amount of gas leakingfrom the patient circuit at that pressure; a period of stable pressureduring which there is no flow into or out of the patient's lungs eventhough there is leakage flow in the ventilator tubing system. Therespiratory mechanics maneuver operation 308 may further include usingleak-compensated values of lung flow and lung volume when determiningthe respiratory mechanics.

The newly determined values of lung compliance and lung resistance maybe averaged, low-pass filtered or otherwise combined with the previouslydetermined values. These revised values are then stored for use in laterdelivery of PA ventilation and the ventilator returns to providing PAventilation to the patient.

FIG. 4 illustrates an embodiment of a method for providing aleak-compensated PA breath to a patient. In an embodiment, the method400 corresponds to the operations performed during the PA ventilationoperation 306 and the respiratory mechanics maneuver operation 308discussed with reference to FIG. 3. In the embodiment of the method 400illustrated, the operations occur repeatedly while the ventilator isproviding PA ventilation, such as once a sample period or computationcycle.

During PA ventilation, the pressure and flow and other parameters of thesystem are monitored, illustrated by the monitoring operation 402. In anembodiment, the monitoring operation 402 collects data including theinstantaneous pressure and/or flow at or indicative of one or morelocations in the ventilation tubing system. Depending upon how aparticular leak model is defined, the operation 402 may also includemaking one or more calculations using data from pressure and flowmeasurements taken by the sensors. For example, a model may require aflow measurement as observed at the patient interface even though theventilation system may not have a flow sensor at that location in theventilation tubing system. Thus, a measurement from a sensor or sensorslocated elsewhere in the system (or data from a different type of sensorat the location) may be mathematically manipulated in order to obtain anestimate of the flow observed at the patient interface in order tocalculate the leak using the model.

The data obtained in the monitoring operation 402 is then used tocalculate leakage from the ventilator tubing system in a leakagecalculation operation 404. In an embodiment, the leakage calculationoperation 404 uses the data obtained in the monitoring operation 402,e.g., some or all of the instantaneous pressure and flow data collectedduring the monitoring operation 402 as well as information about thecurrent respiratory phase (inhalation or exhalation).

The leakage calculation operation 404 calculates an instantaneousleakage flow or volume for the sample period. The instantaneous leakageis calculated using a mathematical formula that has been previouslydetermined. Any leakage model, now known or later developed, may beused. In an embodiment, the mathematical formula is a leakage model thatseparates the leak into the sum of two leak components, inelastic leakand elastic leak, in which each component represents a differentrelationship between the quantity of leakage from the ventilation systemand the measured current/instantaneous pressure and/or flow of gas inthe ventilation system. As discussed above, the inelastic leak may bemodeled as the flow through a rigid orifice of a fixed size while theelastic leak may be modeled as the flow through a different orifice of asize that changes based on the pressure (or flow) of the gas in theventilation system.

An example of a method and system for modeling leak in a ventilationsystem as a combination of an elastic leak component and an inelasticleak component can be found in commonly-assigned U.S. Provisional PatentApplication Ser. No. 61/041,070, filed Mar. 31, 2008, titled VENTILATORLEAK COMPENSATION, which application is hereby incorporated by referenceherein. The VENTILATOR LEAK COMPENSATION represents one way ofcharacterizing the leak from a ventilation system as a combination ofelastic and inelastic components. Other methods and models are alsopossible and may be adapted for use with this technology.

The mathematical formula used to calculate leakage may contain severalparameters that are empirically determined and that may be periodicallyor occasionally revised in order to maintain the accuracy of the leakageestimate. For example, in an embodiment the parameters of a leakageformula include a first constant associated with the rigid orifice and asecond constant associated with the variable-sized orifice. At varioustimes during ventilation, the calculated leakage may be checked againsta measured leakage and, if the estimate is significantly different fromthe measured leakage, the constants may be revised. This revision of theparameters in a leakage formula may be done as part of the leakagecalculation operation 404 or may be done as a separate operation (notshown) that may, or may not, be performed every sample period.

The term instantaneous is used herein to describe a determination madefor any particular instant or sampling period based on the measured datafor that instant. For example, if a pressure measurement is taken every5 milliseconds (sample period), the pressure measurement and the leakagemodel can be used to determine an instantaneous leak flow based on theinstantaneous pressure measurement. With knowledge of the length of thesample period, the instantaneous flow may then be used to determine aninstantaneous volume of gas leaking out of the circuit during thatsample period. For longer periods covering multiple sample periods theinstantaneous values for each sample period may be summed to obtain atotal leakage volume. If a measurement is also the most recentmeasurement taken, then the instantaneous value may also be referred toas the current value.

After the current leak has been calculated, the method 400 furtherestimates the leak-compensated instantaneous lung flow to or from thepatient in a lung flow estimation operation 406. The estimated lung flowis compensated for the leak flow calculated in the instantaneous leakcalculation operation 404 so that it represents a more accurate estimateof the actual flow into (or out of depending on the point of view andperiod selected) the lungs of the patient.

In the embodiment illustrated, the leak-compensated net lung volume isalso calculated as part of the lung flow estimation operation 406. In anembodiment, this may be performed by maintaining a running summation ofnet flow into/out of the lung over the period of a breath. For example,upon triggering inhalation, the ventilator may set a variablecorresponding to net lung volume to zero and, each sample period, updatethis net lung volume to include the detected leak-compensatedinstantaneous lung flow delivered to the patient during that sampleperiod.

In the PA ventilation method 400 illustrated, the leak-compensated lungflow or net volume is then used along with the support setting and thepreviously determined lung resistance and lung compliance (which werethemselves determined using leak-compensated lung flows and net lungvolumes) to calculate the amount of assistance to provide duringventilation.

The PA ventilation method 400 also includes calculating the patienteffort in effort calculation operation 408. As described above, theoperation 408 may use one or more equations that relate the amount ofpressure to calculate the patient's effort based on the instantaneouslung flow, current lung volume, lung compliance, lung resistance,support setting and other factors such as circuit compliance andresistance. Various methods are known in the art for calculating patienteffort that use one or more respiratory mechanics and other parametersmeasurable while providing ventilatory support. Any such method fordetermining patient effort, now known or later developed, may be usedherein.

The appropriate amount of ventilation is then provided in a ventilationoperation 410. In this operation, the patient effort calculated aboveand the support setting are used to determine how much assistance toprovide. Of course, if there is no patient effort (i.e., during theexpiratory phase), no ventilation is provided. In this case, theappropriate ventilation may be providing a predetermined positive endexpiratory pressure (PEEP) level. Depending on the equations used, theeffort calculation operation 408 may not be technically necessary as theamount of ventilation to provide may be determined directly from themonitored data and the previously determined values of lung complianceand resistance using an appropriate algorithm.

The PA ventilation method 400 also periodically determines therespiratory mechanics from the leak-compensated lung parameters, whichis illustrated by the determination operation 412. For example, in anembodiment respiratory mechanics may be calculated on a fixed or randomschedule or calculated in response to an explicit operator command. Inaddition, depending on the respiratory mechanics determination methodused, there may be a requirement that the respiratory mechanicscalculation (using data collected during the breath) be performed at acertain point within the patient's respiration such as at the end of theinspiratory phase or the end of the expiratory phase or after thecompletion of a specific maneuver (e.g., an Inspiratory Hold Maneuver).The respiratory mechanics calculation includes the performance arespiratory mechanics “maneuver,” that is a specified set of controlledactions on the part of the ventilator. In an embodiment, the maneuverincludes interrupting the therapeutic delivery of respiratory gas for aperiod of time and monitoring and/or changing the pressure and flow, sothat data concerning the response of the patient's lung to thecontrolled actions may be obtained. An example of a respiratorymechanics maneuver is provided below with reference to the staticdetermination of lung compliance.

If it is time to calculate respiratory mechanics, a calculaterespiratory mechanics operation 414 is performed. The calculaterespiratory mechanics operation 414 may also include performing theappropriate maneuver, as necessary to obtain the data for therespiratory mechanics model being used. In the operation 414, thenecessary data for the respiratory mechanics model being used isobtained and the respiratory mechanics parameters are estimated.

In the calculate respiratory mechanics operation 414 leak-compensatedvalues are used to estimate the respiratory mechanics. For example, ifthe respiratory mechanics model being used requires a total deliveredlung volume, the leak-compensated lung volume is used. Likewise, if aninstantaneous lung flow is required, a leak-compensated instantaneouslung flow is used in generating the estimate.

The method 400 is then repeated every computational cycle or sampleperiod, as illustrated by the feedback loop, so that the instantaneouslung flow and net lung flow are continuously determined and, whenappropriate, the respiratory mechanics are recalculated based on thecurrent leak-compensated data. In an alternative embodiment, theleak-compensation of lung flow and net delivered lung volume may beperformed as part of the calculate respiratory mechanics operation 414in order to reduce the number of calculations a processor must performevery cycle.

The following is a discussion of two embodiments of methods forcompensating the estimation of respiratory mechanics in the presence ofleaks. The first embodiment is that of applying leak compensation to astatic compliance and resistance determination. The second embodiment isthat of applying leak compensation to a dynamic compliancedetermination.

Leak-Compensation of Static Determination of Lung Compliance

FIG. 5 illustrates an embodiment of a method for estimating respiratorymechanics of patient that utilizes a respiratory mechanics maneuver. Inthe embodiment shown, the ventilator is providing respiratory gas to apatient in accordance with some mode of operation such as a mandatorymode or a pressure assist mode (e.g., a mandatory volume-controlled(VCV) inspiration under square waveform setting or a mandatorypressure-controlled (PCV) inspiration with specific settings, or executean inspiratory hold at the end of a PA inspiration), as is well known inthe art. During operation, an operator command to estimate thecompliance of the patient is received in a receive command operation502. In an embodiment, the command may be entered by the operator of theventilator via selection of a button or other interface element on auser interface of the ventilator. In an alternative embodiment, theventilator may perform the method 500 automatically such asperiodically, randomly or upon the detection of predeterminedrespiratory event.

In the embodiment shown, the system performs a respiratory maneuverwhich includes the forced imposition of a stable period at the end of aninspiratory phase so that there is no flow delivery to or from thepatient's lung. The method 500 includes a delay until the next end of aninspiratory phase is detected, as illustrated by detect end ofinspiratory phase operation 504.

When the end of the inspiratory phase is detected, a stabilizationoperation 506 is performed. In an embodiment, the operation 506 includesstabilizing the pressure and flow in the patient circuit so that thereis no flow into or out of the patient's lungs at the point in which thelungs have taken a breath and thus are expanded with a known volume ofgas (as determined during normal operation of the ventilator asdiscussed above with reference to the leak-compensated lung volume).

The stabilization operation 506 and the maintain stable conditionoperation 508 (discussed below) may sometimes be referred tocollectively as a pause maneuver or a plateau maneuver. They areseparated in this discussion for clarity purposes.

In order to stabilize the pressure and flow to achieve no flow betweenthe circuit and the patient, if there are leaks in the tubing systemthese leaks are compensated for by the ventilator. Thus, in order tostabilize the flow the ventilator provides a leak-compensation flow thatis equal to the amount of leakage from the system estimated by theleakage model at the stable pressure.

In practice, the stabilization of the pressure and flow is an iterativeprocess in which the ventilator monitors the pressure and adjustsdelivered flow until the pressure and flow stabilize at the point wherethe pressure and flow correspond to a solution to the leak model and thelung flow is practically zero, i.e., the current flow provided by theventilator is the leakage flow determined from the model using thecurrent pressure. In an embodiment, for a flow and pressure to beconsidered stable, a certain acceptable error may be allowed between thecalculated leakage (calculated based on the current pressure) and theactual measured flow. Such an error may be predetermined amount or rangebased on an absolute difference between delivery and calculated flow orpressure or relative difference (e.g., calculated flow within x % ofactual stable flow at a given pressure). Various methods for stabilizingpressure and flow in a ventilation tubing system are known in the artand any suitable method may be adapted for use in conjunction with thetechnology described in this disclosure.

When attempting to stabilize the pressure and flow during thestabilization operation 506, the leakage model may be used to increasethe speed of the stabilization through a prediction of the likelyresulting leakage flow at different pressures. This information may beused to determine a more accurate initial starting point for thestabilization and determine more accurate selection of adjustments to bemade in order to more quickly converge on the stable pressure and flow.

After a stable pressure has been achieved, in the embodiment shown amaintain stable condition operation 508 is performed. The maintainstable condition operation 508 may maintain the stable pressure andstable, leak-compensating flow for a predetermined period of time suchas 25-200 milliseconds or more preferably between 50-100 milliseconds.During the operation 458, the drop in pressure over the period of themaneuver may be monitored to ensure that it is within some acceptableperformance threshold. If it is not, the ventilator may resumeattempting to stabilize the pressure and flow or may about the methodand attempt the method 500 at the end of the next inspiratory phase.

The stable pressure observed during the maintain stable conditionoperation 508 is then used to calculate the leak-compensated compliancein a lung compliance calculation operation 510. The pressure value usedmay be an actual pressure or an average pressure observed over themaneuver period. Alternatively, different values derived from or basedon the observed stable pressure may be calculated and used depending onthe data required by the particular respiratory mechanics model beingutilized.

In addition to using the stable pressure obtained during the pausemaneuver, the compliance calculation operation 510 further utilizesleak-compensated lung flow and leak-compensated net lung volume whenperforming the calculation.

As discussed above, any suitable model for calculating lung compliancemay be used. For example, in an embodiment of the compliance calculationoperation 510 compliance is calculated using the following simple model:

Stable pressure=Leak-Compensated Net Lung Volume/Compliance.

or, stated a different way,

Compliance=Leak-Compensated Net Lung Volume/Stable pressure.

By using a leak-compensated value for lung volume and a stable pressuredetermined while compensating for leaks in the patient circuit, a moreaccurate leak-compensated lung compliance is estimated.

The leak-compensated compliance then may be used in a subsequentoperation to determine a leak-compensated lung resistance, In theembodiment of the method 500 shown, this is illustrated by optionalresistance calculation operation 512. In an embodiment of the resistancecalculation operation 512 after the leak-compensated compliance has beendetermined, a resistance model that calculates lung resistance based onpressure, flow and compliance may be used to calculate resistance. Anexample of one such resistance model is as follows:

P(t ₂)−P(t ₁)=(V(t ₂)−V(t ₁))/C+R*(Q(t ₂)−Q(t ₁))

In which t₁ and t₂ are different times during a breath, P(t) is theairway pressure at time t, V(t) is the delivered lung volume at time t,C is the lung compliance, R is the lung resistance and Q(t) is the netlung flow at time t. In the resistance model provided above,leak-compensated lung flow, leak-compensated net lung volume andleak-compensated lung compliance are utilized to obtain aleak-compensated resistance. This computation may be performedrepeatedly over several appropriate time windows and combined together(e.g., by an averaging method) to generate an estimate for lungresistance. Also, lung resistance may be determined from theleak-compensated exhalation flow waveform subsequent to the inspiratorypause maneuver using algorithms for resistance estimation under no leakconditions.

In an alternative embodiment of method 500, if it is determined that theventilator has relatively low leakage, the lung compliance calculationoperation 510 may forego the use of leak-compensated lung flow and netlung volume but still utilize the stable pressure determined through theprovision of a leak-compensating flow during the pause maneuver. Lungcompliance calculated in this fashion is still consideredleak-compensated as the stable pressure was determined by compensatingfor leakage during the pause maneuver when generating the stablepressure.

Leak-Compensation of Dynamic Determination of Lung Compliance andResistance

FIG. 6 illustrates an embodiment of a method for dynamically estimatingrespiratory mechanics of patient. This may be used as an alternativemethod of calculating lung compliance and resistance, thereby obviatingthe need to perform respiratory maneuvers during PA ventilation.

In the embodiment shown, the ventilator is providing respiratory gas toa patient in accordance with some mode of operation such as a mandatorymode or a pressure assist or support or PA mode, as is well known in theart. During operation, the ventilator detects a condition that indicatesthat it is time to estimate the respiratory mechanics of the patient.This is illustrated by the detection operation 602. In an embodiment,the condition detected may be a command entered by the operator of theventilator via selection of a button or other interface element on auser interface of the ventilator. Alternatively, the ventilator mayperform the dynamic estimation automatically such as during everybreath, after a predetermined period of time or after the detection ofsome occurrence such as once every 100 breaths or upon detection ofcertain flow conditions.

Following determination that it is time to calculate the dynamicrespiratory mechanics, the method 600 retrieves and/or calculatesleak-compensated lung flow and leak-compensated net lung volume asnecessary depending on whether the leak-compensated data already existsor not. For example, in an embodiment the leak-compensated lung flowsfor each sampling period may be available but the leak-compensated netlung volume may only be available “as needed” by calculating it from thecompensated lung flow data.

The method 600 then calculates the leak-compensated respiratorymechanics in a calculation operation 606. The leak-compensatedrespiratory mechanics are calculated from a predetermined dynamicrespiratory mechanics model using the leak-compensated lung flows,leak-compensated net lung volume(s) and pressure in order to obtainestimates of dynamic compliance and dynamic resistance that arecompensated for the leaks in the tubing system. The method 600 is thenrepeated as necessary.

Any respiratory mechanics model may be used as long as the model may beadapted to be used in a dynamic calculation, that is withoutinterrupting the ventilation of the patient. Many such models are knownin the art, some requiring iterative solutions of a set of multipleequations using data obtained over of a period of time.

FIG. 7 illustrates a functional block diagram of modules and othercomponents that may be used in an embodiment of ventilator thatcompensates for elastic and rigid orifice sources of leaks whendetermining patient respiratory mechanics. In the embodiment shown, theventilator 700 includes pressure sensors 706 (two are shown placed atdifferent locations in the system), flow sensors (one is shown), and aventilator control system 702. The ventilator control system 702controls the operation of the ventilator and includes a plurality ofmodules described by their function. In the embodiment shown, theventilator control system 702 includes a processor 708, memory 714 whichmay include mass storage as described above, a leak estimation module712 incorporating a parametric leak model accounting for both elasticand rigid orifice leak sources such as that described in U.S.Provisional Application 61/041,070 previously incorporated herein, aleak-compensated static respiratory mechanics module 716, a pressure andflow control module 718, a monitoring module 722, a leak model module720, a leak-compensated dynamic respiratory mechanics module 724, and aleak-compensated lung flow and volume estimation module 726. Theprocessor 708 and memory 716 have been discussed above. Each of theother modules will be discussed in turn below.

The main functions of the ventilator such as receiving and interpretingoperator inputs and providing therapy via changing pressure and flow ofgas in the ventilator circuit are performed by the control module 718.In the context of the methods and systems described herein, the module718 will perform one or more actions upon the determination that apatient receiving therapy is inhaling or exhaling.

In the embodiment described herein, the control module 718 determinesand provides the appropriate amount of ventilation when in PAventilation mode. This may include calculating patient effort and, basedon the patient effort and the support setting, determining theappropriate amount of ventilation, i.e., the pressure and/or flow toprovide to the patient. This may include performing one or morecalculations based on leak-compensated lung flow, leak-compensated lungvolume, leak-compensated lung compliance and leak-compensated lungresistance. PA ventilation is based on an estimation of patient'srespiratory effort, therefore, patient effort may be first calculatedand then the amount of ventilation (desired pressure reference)calculated therefrom.

The static calculation of respiratory mechanics is performed by theleak-compensated static respiratory mechanics module 716. The module 716utilizes one or more respiratory models suitable for staticdetermination of respiratory mechanics and one or more embodiments ofthe method 400 described above to calculate leak-compensated respiratorymechanics such as lung compliance and lung resistance. The module 716uses leak-compensated values for one or both of lung flows and net lungvolume. Leak-compensated values may be retrieved if they have alreadybeen calculated or may be calculated as needed from leakage informationreceived from the leak-compensated lung flow and net lung volumeestimation module 726. When calculating static respiratory mechanics,the module 716 may control the operation of the ventilator so that apause maneuver is performed when required. Alternatively, some or all ofthe actions required in a pause maneuver may be controlled by thecontrol module 718 in response to a respiratory mechanics calculationrequest and the data obtained during the maneuver provided to the staticrespiratory mechanics module 716.

The dynamic calculation of respiratory mechanics is performed by theleak-compensated dynamic respiratory mechanics module 724. The module724 utilizes one or more dynamic respiratory models and one or moreembodiments of the method 500 described above to calculateleak-compensated respiratory mechanics such as lung compliance and lungresistance. The module 724 uses leak-compensated values for one or bothof lung flows and net lung volume. Leak-compensated values may beretrieved if they have already been calculated or may be calculated fromleakage information received from the leak-compensated lung flow and netlung volume estimation module 726.

The current conditions in the ventilation system are monitored by themonitoring module 722. This module 722 collects the data generated bythe sensors 704, 706 and may also perform certain calculations on thedata to make the data more readily usable by other modules or mayprocess the current data and or previously acquired data or operatorinput to derive auxiliary parameters or attributes of interest. In anembodiment, the monitoring module 722 receives data and provides it toeach of the other modules in the ventilator control system 702 that needthe current pressure or flow data for the system.

In the embodiment shown, compensated lung flows are calculated by thelung flow module 726. The lung flow module 726 uses a quantitative modelfor lung flow of the patient during both inhalation and exhalation andfrom this characterization and pressure and flow measurements generatesan estimate for instantaneous lung flow. In an embodiment, lung flow maybe simply determined based on subtracting the estimated leak flow andmeasured outflow via the expiratory limb from the flow into theinspiratory limb, thereby generating a leak-compensated net flow into(or out of) the lung. The lung flow module 726 may or may not alsocalculate an ongoing leak-compensated net lung volume during a patient'sbreath as described above. Compression in the circuits and accessoriesmay also be accounted for to improve the accuracy of estimated lungflow.

The leak model parameters are generated by the leak estimation module712 which creates one or more quantitative mathematical models,equations or correlations that uses pressure and flow observed in theventilation system over regular periods of respiratory cycles(inhalation and exhalation) and apply physical and mathematicalprinciples derived from mass balance and characteristic waveformsettings of ventilation modalities (regulated pressure or flowtrajectories) to derive the parameters of the leak model incorporatingboth rigid and elastic (variable pressure-dependent) orifices. In anembodiment, the mathematical model may be a model such as:

Q _(inelastic) =R ₁ *P _(i) ^(x)

Q _(elastic) =R ₂ *P _(i) ^(y)

wherein Q_(elastic) is the instantaneous leak flow due to elastic leaksin the ventilation system, Q_(inelastic) is the instantaneous leak flowdue to inelastic leaks in the ventilation system, R₁ is the inelasticleak constant, R₂ is the elastic leak constant, P_(i) is the current orinstantaneous pressure measurement, x is an exponent for use whendetermining the inelastic leak and y is an exponent different than x foruse when determining the elastic leak. The group R₁*P_(i) ^(x)represents flow through an orifice of fixed size as a function ofinstantaneous pressure P_(i) and the group R₂*P_(i) ^(y) represents flowthrough a different orifice that varies in size based on theinstantaneous pressure. The equations above presuppose that there willalways be an elastic component and an inelastic component of leakagefrom the ventilation system. In the absence of an elastic component or aleak source of varying size, R₂ would turn out be zero.

In the embodiment shown, the current or instantaneous elastic leak iscalculated by the leak estimation module 712. The calculation is madeusing the elastic leak portion of the leak model developed by the leakestimation module 712 and the pressure data obtained by the monitoringmodule 722. The leak estimation module 712 may calculate a newinstantaneous elastic leak flow or volume for each pressure sample taken(i.e., for each sampling period) by the monitoring module 722. Thecalculated elastic leak may then be provided to any other module asneeded.

In the embodiment shown, the current or instantaneous inelastic leak isalso calculated by the leak estimation module 712. The calculation ismade using the inelastic leak portion of the leak model and the pressuredata obtained by the monitoring module 722. The leak estimation module712 may calculate a new instantaneous inelastic leak flow or volume foreach pressure sample taken (i.e., for each sampling period) by themonitoring module 722. The calculated inelastic leak may then beprovided to any other module as needed.

The system 700 illustrated will compensate lung flow for leaks due toelastic and inelastic leaks in the ventilation system. Furthermore, thesystem may perform a dynamic compensation of lung flow based on thechanging leak conditions of the ventilation system and the instantaneouspressure and flow measurements. The system then compensates therespiratory mechanics calculations based on the estimated leakage in thesystem. By compensating for the inelastic as well as the elasticcomponents of dynamic leaks, the medical ventilator can more accuratelyand precisely estimate the respiratory mechanics of a patient includingestimating the lung compliance and lung resistance.

It will be clear that the systems and methods described herein are welladapted to attain the ends and advantages mentioned as well as thoseinherent therein. Those skilled in the art will recognize that themethods and systems within this specification may be implemented in manymanners and as such is not to be limited by the foregoing exemplifiedembodiments and examples. For example, the operations and steps of theembodiments of methods described herein may be combined or the sequenceof the operations may be changed while still achieving the goals of thetechnology. In addition, specific functions and/or actions may also beallocated in such as a way as to be performed by a different module ormethod step without deviating from the overall disclosure. In otherwords, functional elements being performed by a single or multiplecomponents, in various combinations of hardware and software, andindividual functions can be distributed among software applications. Inthis regard, any number of the features of the different embodimentsdescribed herein may be combined into one single embodiment andalternate embodiments having fewer than or more than all of the featuresherein described are possible.

While various embodiments have been described for purposes of thisdisclosure, various changes and modifications may be made which are wellwithin the scope of the technology described herein. For example, thesystems and methods described herein could be adapted to automaticallydetermine leak-compensated resistance and/or compliance and initiate analarm if the leak-compensated values are outside of a specified rangefor predetermined leakage values, thus eliminating false resistance andcompliance alarms due to changes in leakage. Numerous other changes maybe made which will readily suggest themselves to those skilled in theart and which are encompassed in the spirit of the disclosure and asdefined in the appended claims.

1. A method of compensating for leakage in a ventilation system duringdelivery of proportional assist ventilation to a patient comprising:monitoring an instantaneous flow in the ventilation system based on oneor more measurements of pressure and flow in the ventilation system;modeling leakage from the ventilation system as a first leakagecomponent through a first orifice of a fixed size and a second leakagecomponent through a second orifice of a varying size, wherein the firstand second leakage components are different functions of instantaneouspressure in the ventilation system; estimating a leak-compensatedinstantaneous lung flow of gas inhaled or exhaled by the patient basedon the one or more measurements, the first leakage component and secondleakage component; using the leak-compensated lung flow and apredetermined respiratory mechanics model to estimate a leak-compensatedlung compliance and a leak-compensated lung resistance; and calculatinga pressure to be delivered to the patient based on the leak-compensatedlung flow, the leak-compensated lung compliance and the leak-compensatedlung resistance.
 2. The method of claim 1 wherein the method uses adynamic respiratory mechanics model and the method further comprises:detecting a condition indicating that leak-compensated lung complianceshould be calculated; retrieving one or more previously estimatedleak-compensated instantaneous lung flows; and estimating aleak-compensated lung compliance and a leak-compensated lung resistanceusing the dynamic respiratory mechanics model and the retrieved one ormore previously estimated leak-compensated instantaneous lung flows. 3.The method of claim 2 wherein the dynamic respiratory mechanics modelrequires input of a lung volume change over a predetermined time periodand the method further comprises: calculating a leak-compensated lungvolume change for the predetermined time period based onleak-compensated lung flows associated with the predetermined timeperiod.
 4. The method of claim 1 wherein the method calculates at leastthe leak-compensated lung compliance and the method further comprises:calculating a leak-compensated lung volume based on the leak-compensatedlung flow during an inspiratory phase; stabilizing the delivery of gasso that the medical ventilator delivers a stable flow of gas at a stablepressure, wherein the stable flow and stable pressure are determinedbased on the first leakage component and second leakage component at thestable pressure; maintaining the stable flow of gas at the stablepressure for at least a predetermined time interval; and calculating theleak-compensated lung compliance based on the leak-compensated lungvolume and the stable pressure.
 5. The method of claim 4 whereinstabilizing further comprises: stabilizing the delivery of gas byadjusting the instantaneous flow of gas to be within a predeterminedamount of the first leakage component and second leakage component atthe instantaneous pressure.
 6. The method of claim 4 wherein stabilizingfurther comprises: stabilizing the delivery of gas so that the lung flowis practically zero.
 7. The method of claim 4 further comprising:calculating a leak-compensated lung resistance based on theleak-compensated lung compliance and one or more of a previouslycalculated leak-compensated lung flow, a previously calculatedleak-compensated lung volume and a previously measured pressure.
 8. Themethod of claim 4 wherein the stabilizing and maintaining operations areperformed at the end of the inspiratory phase.
 9. The method of claim 1wherein the method calculates at least the lung compliance and themethod further comprises: calculating a leak-compensated lung volumebased on the leak-compensated lung flow during an inspiratory phase andexpiratory phase; stabilizing the delivery of gas so that the medicalventilator delivers a stable flow of gas at a stable pressure, whereinthe stable flow and stable pressure are determined based on the firstleakage component and second leakage component at the stable pressure;maintaining the stable flow of gas at the stable pressure for at least apredetermined time interval; and calculating the leak-compensated lungcompliance based on the leak-compensated lung volume and the stablepressure.
 10. The method of claim 1 wherein the method first calculatesa leak-compensated lung compliance and then, based on theleak-compensated lung compliance, calculates a leak-compensated lungresistance.
 11. A method of compensating for leakage in a ventilationtubing system during delivery of gas from a medical ventilator to apatient comprising: receiving a support setting identifying an amount ofproportional assistance to provide to the patient; identifying aninelastic leakage in the ventilation tubing system as a first functionof at least one of a pressure measurement and a flow measurement in theventilation tubing system; identifying an elastic leakage in theventilation tubing system as a second function of at least one of thepressure measurement and the flow measurement in the ventilation tubingsystem; estimating circuit compliance and circuit resistance of theventilation tubing system; estimating a lung compliance of the patientand a lung resistance of the patient based on the inelastic leakage, theelastic leakage, the circuit compliance, circuit resistance and the atleast one of the pressure measurement and the flow measurement in theventilation tubing system; delivering ventilation to the patient basedon patient effort and the support setting, wherein the patient effort isdetermined from the inelastic leakage, the elastic leakage, the lungcompliance, the lung resistance and the at least one of the pressuremeasurement and the flow measurement in the ventilation tubing system.12. The method of claim 11 wherein estimating lung compliance furthercomprises: generating a plurality of leak-compensated lung flowsassociated with a period of time based on the elastic leakage and atleast one of pressure measurements and the flow measurements associatedwith the period of time; generating a leak-compensated net lung volumefor the period of time based on the plurality of leak-compensated lungflows; and calculating lung compliance using the leak-compensated netlung volume.
 13. The method of claim 11 wherein estimating lungcompliance and lung resistance further comprises: generating a pluralityof leak-compensated lung flows associated with a period of time based onthe elastic leakage and at least one of pressure measurements and theflow measurements associated with the period of time; generating aleak-compensated net lung volume for the period of time based on theplurality of leak-compensated lung flows; and calculating lungcompliance and lung resistance using the leak-compensated net lungvolume, the plurality of leak-compensated lung flows for the period oftime and the pressure measurements associated with the period of time.14. A pressure support system comprising: a pressure generating systemadapted to generate a flow of breathing gas; a ventilation tubing systemincluding a patient interface device for connecting the pressuregenerating system to a patient; one or more sensors operatively coupledto the pressure generating system or the ventilation tubing system, eachsensor capable of generating an output indicative of a pressure of thebreathing gas; a leak estimation module that identifies leakage in theventilation tubing system; a respiratory mechanics calculation modulethat generates a leak-compensated lung compliance and a leak-compensatedlung resistance based on the leakage and at least one output indicativeof a pressure of the breathing gas; and a proportional assistanceventilation module that causes the pressure generating system to provideventilation to the patient based on patient effort and a supportsetting, wherein the patient effort is determined from the leakage, theleak-compensated lung compliance and the leak-compensated lungresistance.
 15. The system of claim 14 wherein the respiratory mechanicsmodule is a static respiratory mechanics module that uses a staticrespiratory mechanics model to determine a leak-compensated static lungcompliance and a leak-compensated static lung resistance based on aleak-compensated lung flow and an output of at least one sensor.
 16. Thesystem of claim 14 wherein the respiratory mechanics module is a dynamicrespiratory mechanics module that uses a dynamic respiratory mechanicsmodel to determine at least one of a leak-compensated dynamic lungcompliance and a leak-compensated dynamic lung resistance based on theleak-compensated lung flow and an output of at least one sensor.
 17. Thesystem of claim 14 wherein the leak estimation module further identifiesan inelastic leakage in the ventilation tubing system and wherein thecompensation module further generates the leak-compensated lung flowbased on the inelastic leakage.
 18. The system of claim 17 furthercomprising: a pressure monitoring module that monitors at least oneoutput of the one or more sensors and provides data indicative of thepressure of the breathing gas to the leak estimation module.
 19. Acontroller for a medical ventilator comprising: a microprocessor; amodule that compensates calculations of lung compliance and lungresistance based on instantaneous elastic leakage and instantaneousinelastic leakage of breathing gas from a ventilation tubing system; anda pressure generating module controlled by the microprocessor thatprovides proportional assist ventilation based on the compensated lungcompliance and lung resistance.
 20. The controller of claim 19 furthercomprising: a leak estimation module that identifies the instantaneousinelastic leakage from the associated ventilation tubing system and theinstantaneous elastic leakage from the associated ventilation tubingsystem.